The present disclosure generally relates to plasma ashing processes for selectively removing photoresist, organic overlayers, and polymer residues from a substrate surface, and in particular, processes for monitoring oxygen and/or nitrogen species in a substantially oxygen and nitrogen free plasma.
Ashing is a plasma mediated stripping process by which photoresist, organic overlayers, and/or polymer residues are stripped or removed from a substrate upon exposure to the plasma. Ashing generally occurs after an etching process has been performed in which the photoresist material is used as a photomask for etching a pattern into the substrate. The ashing process is also used to remove other organic layers such as the anti-reflection coating (ARC), if present. Additionally, the ashing process may be performed for removal of misaligned resist patterns (“rework wafers”) and in lift-off processes. It is well known that the process steps occurring prior to ashing may modify the surface of the photoresist and ARC, and/or form polymers/residues. It is highly desirable when ashing that complete removal of the photoresist and other organic overlayers, polymers/residues occur as quickly as possible without loss of any of the materials comprising the underlayers and/or the materials that form the substrate.
It is important to note that ashing processes significantly differ from etching processes. Although both processes may be plasma mediated, an etching process is markedly different in that the plasma chemistry is deliberately chosen to permanently transfer an image into the substrate by removing portions of the substrate surface through openings in a photoresist mask. This type of plasma generally includes high-energy ion bombardment at low temperatures to remove selected portions of the substrate. Moreover, the portions of the substrate exposed to the high-energy ions are generally removed at a rate equal to or greater than the removal rate of the photoresist mask.
In contrast, ashing processes generally refer to selectively removing the photoresist mask and any polymers or residues formed during etching without removing portions of the underlying substrate. The ashing plasma chemistry is much less aggressive than etching chemistries and generally is chosen to remove the photoresist mask layer at a rate much greater than the removal rate of the underlying substrate. Moreover, most ashing processes heat the substrate to temperatures greater than 200° C. to increase the plasma reactivity. Thus, etching and ashing processes are directed to removal of significantly different materials and as such, require completely different plasma chemistries and processes. Successful ashing processes are not used to permanently transfer an image into the substrate. Rather, successful ashing processes are defined by the photoresist, polymer and residue removal rates without affecting and/or removing layers comprising the underlying substrate.
Ashing selectivity is defined as the relative removal rate of the photoresist and other organic overlayers, compared to the underlying layer. It is generally preferred to have an ashing selectivity of at least 20:1, wherein at least 20 times as much photoresist is removed as the underlying substrate. More preferably, the ashing selectivity is much greater than 100:1.
During plasma ashing processes, it is important to maintain a critical dimension (CD) for the various features within a tightly controlled specification as well as promote proper underlayer surface conditions for successful metal filling in the process steps occurring after photoresist and/or polymer/residue removal. Small deviations in the patterned profiles formed in the underlayers can adversely impact device performance, yield and reliability of the final integrated circuit. Traditionally, the ashing plasma has been generated from substantially oxygen and/or nitrogen containing gases. However, it has been found that these oxygen and/or nitrogen containing plasmas readily damage certain materials used in advanced integrated circuit manufacture. For example, oxygen-containing plasmas are known to raise the dielectric constant of low k dielectric underlayers during plasma processing. The increases in dielectric constant affects, among others, interconnect capacitance, which directly impacts device performance. Moreover, the use of oxygen-containing plasmas is generally less preferred for advanced device fabrication employing copper metal layers since the copper can be oxidized.
In order to overcome these problems, substantially oxygen-free and substantially nitrogen-free ashing plasma chemistries have been developed. By substantially oxygen-free it is generally meant that the plasma chemistry has less than about 50 parts per million (ppm) oxygen in the gas mixture defining the plasma, and by substantially nitrogen-free, it is generally meant that the plasma chemistry has less than about 10 ppm nitrogen in the gas mixture defining the plasma. Though oxygen-free plasma can be used to remove photoresist, it is desirable to use a substantially oxygen-free plasma to more effectively remove photoresist, organic overlayers, and polymers/residues from substrates containing low k dielectric materials without physically damaging the low k dielectric layer. Substantially oxygen-free and substantially nitrogen-free plasmas can be generated from hydrogen and helium gas mixtures, but tend to contain residual nitrogen due to the purity levels of gases generally used, and due to the relaxed standards for leak integrity of vacuum systems typically needed in plasma ashers. It is generally less preferred to have nitrogen present in any substantial quantity, since in some cases, it has been found that the use of plasmas containing nitrogen may alter and/or affect the chemical, mechanical, and electrical properties of the underlying substrate. For example, exposing carbon and/or hydrogen containing low k dielectric materials to plasmas generated from hydrogen, helium gas mixtures (containing substantial amounts of oxygen and/or nitrogen) can result in significant damage to the underlying substrate. Occasionally, the damage is not detected during a visual inspection such as a metrology inspection of the substrate after plasma processing. However, the damage can be readily demonstrated by a subsequent wet cleaning process, as may be typically employed in the integrated circuit manufacturing process undesirably after plasma ashing, wherein portions of the carbon and/or hydrogen-containing low k dielectric material are removed. The removed portions of the dielectric material are a source of variation in the critical dimension (CD) of the substrate feature and are frequently unacceptable, which then impacts overall device performance/yield. Moreover, even if a wet clean process is not included, the electrical and mechanical properties of the dielectric material may be changed by exposure to plasmas that contain substantial amounts of oxygen and/or nitrogen, thereby affecting operating performance. It is believed that carbon is depleted from the dielectric material during the plasma exposure, and the oxygen and/or nitrogen species contained therein damages the dielectric in such a way that it causes problems during subsequent metal filling processes, such as the creation of voids at the bottom of trench structures.
Because of the problems discovered due to the unintended presence of nitrogen found in the gas mixture used for forming the substantially oxygen-free and substantially nitrogen-free plasmas as noted above as well as the sensitivity of the low k materials to the presence of nitrogen radicals and/or oxygen radicals, it is important to accurately monitor these species during plasma processing.
Optical emission spectroscopy is a well-known procedure for trace element detection. However, it is difficult to accurately detect the levels of nitrogen and/or oxygen species in the plasma using the relatively unsophisticated spectrometers that are currently employed in the industry to cost-effectively monitor the specific wavelengths associated with the major emission signal for these species, e.g., N2 at 335-337 nanometers (nm) and O at 777 nm. The low cost, unsophisticated optical emission spectrometers currently used by those in the art are generally unacceptable for detecting the relatively low levels of nitrogen and oxygen because of poor resolution and lack of sensitivity inherent to these small footprint type spectrometers. For example, it is very difficult to differentiate and quantify the concentration of oxygen species at concentrations less than 50 parts per million (ppm) using these spectrometers. More sophisticated spectrometers are generally impractical due to the added costs to the plasma ash equipment, the larger foot-print, the complexity of operation as well as the maintenance, calibration, and integration issues associated with these types of equipment.
FIG. 1 illustrates the optical emission spectra in the range of 750 nm to 800 nm for a hydrogen-helium (He—H2) plasma in the presence of varying levels of oxygen. The 0.01 ppm level of oxygen was estimated based on a curve fit whereas the 10-100 ppm levels of oxygen were prepared by gas mixing with ‘low flow’ mass flow controllers. As can be seen from the graph, the oxygen emission signal at 777 nm can be readily distinguished at amounts greater than 20 ppm. However, at less than 20 ppm, resolution and discrimination between the varying amounts is poor.
FIG. 2 graphically illustrates another example wherein the optical emission spectra in the range of 300 nm to 350 nm for a hydrogen-helium (He—H2) plasma was monitored in the presence of varying levels of nitrogen. As can be seen from the graph, at nitrogen levels less than 10-20 ppm, the resolution and discrimination between the varying amounts is poor and not reliable.
Accordingly, there is a need to have an accurate process for monitoring the amounts of nitrogen and/or oxygen in a substantially oxygen and/or nitrogen free plasma by a commercially viable method such as using commercially available low-cost, unsophisticated optical emission spectrometers so that the plasma ashing process can be monitored to prevent damage to the underlying low k structure while providing a robust process for removing the photoresist and any polymers or residues. Desirably, the process should be capable of accurately detecting oxygen levels less than 50 ppm, and nitrogen levels less than 10 ppm.